The spectroscopic reflectometer is a measuring instrument, which irradiates measurement light to an inspection work such as a film body to be measured and generates a spectrum of interference light of reflection light reflected by a surface thereof and transmitted reflection light which transmits the inside of the inspection work and reflected by an opposite boundary face and gets out of the surface, whereby a film thickness of the inspection work is obtained based on the spectrum for measurement, as shown in Patent Document 1.
In such a spectroscopic reflectometer, it is necessary to measure an optical reflectance of the inspection work. Therefore, conventionally, as shown in FIGS. 1(a), 1(b) and 1(c), light is irradiated from a light source 1′ respectively to a calibration sample SPr having a known and constant reflectance and is also applied to a dark sample SPb which does not have any reflection light at all, and an output value of a photo-detector 2′ is measured in each of the cases. Then, an optical reflectance Rs of the inspection work SPs is calculated from (Equation 1) as following. It is noted that, the reason of using the dark sample SPb for measurement is because the output value of the photo-detector 2′ in a state of having no reflection light represents an offset value and this offset value should be cancelled.
                              R          s                =                                                            I                s                            -                              I                b                                                                    I                r                            -                              I                b                                              ⁢                      R            r                                              [                  Equation          ⁢                                          ⁢          1                ]            
Herein, Is is an output value of the photo-detector 2′ with respect to the inspection work SPs, Ir is an output value of the photo-detector 2′ with respect to the calibration sample SPr, Ib is an output value of the photo-detector 2′ with respect to the dark sample SPb, and Rr is an optical reflectance (known) of the calibration sample SPr.
Patent Document 1: JP2005-003401A